High-frequency signal-translating stage



Jan. 30, 1940. R. FREEMAN 2,138,504

HIGH-FREQUENCY SIGNAL-TRANSLATING STAGE File d oct. 22, 1958 2 Sheets-Sheet 1 V Grid volmqn Grid Voltage 0 +s. 'ktgqz 2e *o-a4 25 I AVG INVENTOR ROBERT L. FREEMAN Flea; I v m I ATTORN EY Jan. 30,- 1940. R, F E M 2,188,504

HIGH-FREQUENCY S IGNAL-TRANSLATING STAGE Filed Oct. '22, 1 938 2 Sheets-Sheet 2 40A 0 O O 36\/0o oo o o 5l \,oooo o n o 0000 BSQ/OOOOOCJOOOO'OO +5 Sc. FIG. I l. RINVENTOR UfiBERT L. FREEMAN B ATTORNEY mama Jan. 30, 1940 HIGH-FREQUENCY SIGNAL-TRANSLATING STAGE Robert L. Freeman, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application October 22, 1938, Serial No. 236,540

12 Claims. This invention relates generally to high-frequency signaltranslating stages and particularly to stages for operation at such high frequencies that the conductance of the input circuit of the stage is an appreciable factor in the response characteristic of the stage.

' The input conductance of conventional vacuum tubes utilized in high-frequency translating stages, such as a stage of high-frequency ampli- -1 0 fication, materially reduces the response of the stage. At frequencies above megac'ycles' or thereabout, the input conductance of a conventional vacuum tube is appreciable and at frequencies higher than 50-megacycles the input conductance rather than the inherent tube and circuit capacitance becomes the limiting factor in the response of the stage. condition is that the maximum impedance which can be developed across the input 'circuit .of the stage at these frequencies is limited by the input conductance of the tube.

It is an object of the present invention to provide an improved high-frequency signal-translating stage which is not subject to the above-' mentioned disadvantages.

It is a further object of the invention to provide a high-frequency signal-translating stage, the response of which is not substantially limited bythe input conductance of the vacuum ,tubes utilized in the stage.

In accordance with the invention, a high-frequency signal-translating stage comprises a vacuum-tube electrodestructure effectively including a plurality of space discharge paths in parallel. Certain of the discharge paths comprise the elements of a conventional vacuum tube including input electrodes having an appreciable positive conductance therebetween. Others of the discharge paths comprise input electrodes including a control electrode and a cathode, to-

gether with means for forming a virtual cathode adjacent the control electrode and between. the control electrode and the cathode, whereby a negative conductance existsv between theinput electrodes. The spacing of the electrodes and their normal operating potentials are so proportioned that the above positive andngative conductances are complementaryover the operating range of the stage. a

'Inone embodiment of the invention, the two discharge paths may be comprised in separate vacuum tubes.- In accordance with the preferred embodiment of the invention, the means for forming the above-mentioned virtual cathode comprises two electrodes betweenwhich the neg- The reason for this atively biased control electrode associated with the virtual cathode is positioned. Positive potentials are applied to the two above-mentioned electrodes to form the virtual cathode, thereby providing a negative conductance between the 5 control electrode and cathode, that is, between input electrodes of the tube. In another preferred embodiment of the invention, the two above-mentioned space discharge paths are comprised in the same vacuum tube, '10

the tube having conventional electrodes disposed in one of the space paths and means for forming the above-mentioned virtual cathode disposed in the other of saidspace paths.

The novel features which are believed to be 15 characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with other and further advantages thereof, will best 20 I be understood by reference to the following specification taken in connection with the accompanying drawings, in which Fig. l is a simplified circuit diagram utilized to derive the input conductance characteristicof a conventional vacu- 5 um tube circuit; Fig. 2 is a fragmentary view of the basic electrode structure of a tube providing a negative input conductance; Figs. 3 and 4 are input conductance graphs of a tube having the electrode structure of Fig. 2 and of a con- 30 ventional vacuum tube, respectively; Fig. 5 is a circuit diagram of a high-frequency signal-translating stage in accordance with the invention utilizing two vacuum tubes; and Figs. '7, 9 and 11- illustrate high-frequency signal-translating 5 stages in accordance with the invention utilizing the unitary tube structures of Figs. 6, 8, and 10, respectively.

The input conductance of conventional vacuum tubes is caused by three factors: (1) the di- 40 electric losses of the tube insulators and the tube base; (2) feedback between the cathode and grid circuits through the grid-cathode capacitance of the tube and the self-inductance of the cathode lead, the latter being common to the output 5 and input circuits; and (3) the transit time ofelectrons through the tube. The input conductance caused by dielectric losses is usually small compared to that caused by feedback through. the grid-cathode capacitance and the self-induc- 50 tance of the cathode lead and also is small as compared to that component of input conductance dependent on the transit time of electrons through the tube.

conductance caused by feedback between the input and outputcircuits of a high-frequency Signal-translating stage utilizing a conventional vacuum tube, reference is made to Fig. 1. Fig. 1' comprises a high-frequency signal-translating stage including a vacuum tube It), input terminals H, H and output terminals l3, H. The grid-cathode capacitance of tube I0 isrepresented by condenser i5, while the inductance of the cath de lead is represented by inductance Hi. The impedance of the output circuit is represented schematically by impedance connected across output terminals l3, I.

.If V is the voltage applied to the input termi nals ll, l2 and i the input current which flows because of this voltage, the following equations are obtained neglecting the components of input conductance due to dielectric losses and to transit time of the tube:

V=i(1/a'wC15 |-a'wL1a) -MwL1 (1) where is is the cathode current and u is the angular frequency.

where V is grid-cathode voltage and gig is the change in cathode current for a change in grid voltage. Therefore V=i(I/iwCis+fiwLis+gkfwLis/iwcis) (3) As the cathode-lead inductance of conventional tubes is usually of the order of 0.04 microhenry, while the grid-cathode capacitance of such tubes is of the order of '7 micromicrofarads, the resonant frequency of these two reactances is about 300 megacycles. Therefore, for frequencies up to 100 megacycles wLie is small compared to l/wCis and the former may be neglected. The input admittance Y of the tube may then be expressed as:

Y=i/ V=iw0i s'/ (1=gk7'wL1e-) (4) Or if rationalized and the term (grains) in the denominator is neglected From Equation 4 the expression for the component of input conductance Gr due to feedback through the grid-cathode capacitance and the inductance of the cathode lead is found to be: G1=g3yi=LmC1s (5) The component of input conductance of a conventional vacuum tube dependent on the transit of electrons through the tube, has been obtained.

where F is a function of the transit time. It'

will be noted that Equations 5 and 6 are of similar form and for this reason it is diflicult experimentally to evaluate Gt and G1 separately. Calculations indicate that they are of comparable magnitude for conventional tubes.

Certain vacuum tube structures under certain operating potentials exhibit a negative input conductance.

The essential tube requirements for this phenomenon to occur is a control'electrode slightly negative with respect to its associated cathode interposed between two electrodes of positive potential. This basic tube structure fora tube having co-axial electrodes, is represented by the fragmentary view of Fig. 2, wherein a control electrode 20, associated with a cathode 2|, is shown interposed between two other electrodes 22 and 23. Experiments with a type of tube equivalent to that of Fig. 2, wherein the first grid 22 was connected to a source of potential of volts, and the anode operated at a potential of +200 voltage, showed that a negative conductance of several micromhos obtained between the grid 20 and the cathode at 6 megacycles with the grid 20 biased to 3 volts. Further, this negative conductance decreased with increasing negative bias on the grid 20. The grid conductance bias characteristic of such a tube is shown in Fig. 3 .while the corresponding characteristic of a conventional vacuum tube conventionally connected in a signal-translating stage is shown in Fig. 4.

It has also been determined experimentally that the negative input conductance of a vacuum. tube, as represented by the characteristic of Fig. 3, varies somewhere between the 1.5 and 2.0 power of the operating frequency. This, of course, is nearly the same law of variation as that of the positive input conductance of conventional tubes, as indicated by Equations 5 and 6 above. It is seen that the characteristic curves of Figs. 3 and 4 are generally similar in form but opposite in polarity. It is, therefore, proposed, in accordance with the present invention, to utilize space discharge paths in parallelhaving different types of characteristics and soto adjust the spacing of the electrodes and their operating potentials that the conductance characteristics of the several space paths are complementary over the operating range of the stage. The component input conductances of such a stage then tend to cancel each other. Therefore, the two types of conductance variations tend to be equal and opposite over the frequency range as well as the grid-bias range of the stage, that is, for all operating conditions of the stage. s

Anarrangement of the type just described isshown in Fig. 5 which comprises a conventional electron discharge device and an electron discharge device having an electrode structure including the basic form illustrated in Fig. 2 to provide a virtual cathode between its control electrode and its cathode. The high-frequency signal-translating stage of Fig. 5 comprises input terminals 21, 28 to which are coupled the primary windingof selector transformer 29, the secondary winding of which is tuned by condenser 30. The input electrodes of each of tubes 25 and 26 are connected across the selector circuit 29, 30 in a conventional manner. A tuned circuit 3|, 32 is included in the common anode circuit of tubes 25, 26, while output terminals 33, 34 are coupled across tuned circuit 3|, 32, Suitable operating potentials are applied to the .electrodes of tubes'25, 26' from sources indicated at +80 and +3 while a suitable amplification control potential is applied to the signal input grids from a suitable source indicated at A. V. C.

In considering the operation of the stage of amplification just described, it will be seen that tube 25 has the conventional type of grid voltage-input conductance characteristic shown in Fig. 4 while tube 26 has the type of characteristic istics are approximately complementaryover a wide range of grid-bias voltage which may be adjusted either manually or by a conventional automatic amplification control system as indicated by A. V. C. Therefore, the resultant input conductance of the high-frequency signal-translating stage is materially reduced and may, in fact, with the proper choice of tubes, be reduced substantially to zero by the operation of tubes 25 and 26 in parallel.

The two parallel space-discharge paths provided by tubes 25 and 26 of Fig. are replaced by a single confposite tube in the embodiment of the invention illustrated in Fig. 6. A signaltranslating stage including the electrode structure of Fig. 6 is shown in Fig. 7. The electrode structure of Fig. 6 comprises a cathode 35, a helical control electrode 36 coaxial with cathode 35 and a cylindrical anode 2T coaxial with cathode 35 and control electrode 35. A suppressor grid 38 connected directly to the cathode 35 by conductor 39 and a screen grid '40 is interposed between control electrode 36 and'anode 31. The electrode structure which has so far been described is that of a conventional vacuum tube,

with the exception that the control electrode 36 is spaced from cathode 35 by a distance greater than usual, and such a structure has a positive input conductance when the tube is utilized in a normal manner.

However, a portion of the electrode structure of Fig. 6 includes also the electrodestructure of Fig. 2 so that, for this portion of the tube, a virtual cathode exists adjacent the control electrode 36 and between the control electrode 36 and the cathode 35. The portion of the tube in which this virtual cathode. is effective tends to provide a negative input conductance. In order to provide the virtual cathode an auxiliary electrode 4| is disposed between control electrode 36 and cathode 35, the auxi1-, iary electrode being adapted to be operated at a potential positive with respect to that of the cathode 35. A second auxiliary electrode 42, axially coextensive with auxiliary electrode 4| and connected to cathode 35, is disposed .between electrode 4| .and cathode 35 in order to reduce the amount of current which would otherwise be drawn by positive electrode 4|. It will be understood that in order to provide a suitable transconductance the control electrode 36 must be located reasonably close to-cathode 35 and,

it maybe found expedient to locate only that portion of electrode-36 which is coextensive with lieved that the operation of the circuit of Fig-7 will be apparent from the description which has been given above with reference to ,Fig. 5 and with reference to the electrodestructure of Fig.

6, the space path including electrodes 4| and 42 corresponding to the tube 25 of Fig. 5 and the space path excluding such electrodes corresponding to'the tube 25 of-"Figb5.

space path of the The tube structure of Fig. 8 and the circuit of Fig. 9'are generally similar to those of Figs. 6 and7; respectively, and similar circuit elements have been given identical reference numerals. The primary difference between the tube structure of Fig. 6 and that of Fig. 8 is that auxiliary electrodes 4| and 42, the effective surfaces of which are restricted to a particularv portion of cathode 35, have been replaced by electrodes 5| and 52, respectively. Electrodes 5| and 52 a are each in the form of a helix, the length of which is approximately equivalent to that of cathode 35, but the pitch ofwhich is such that there are appreciable portions of cathode 35 upon which electrodes 5| and 52 are not eiiective, these portions of the tube providing space discharge paths which are efiective to. provide a positive input conductance in thenormal manner. The tube structure of Fig. 10 and the circuit of Fig. 11 are identical with those of Figs. 8 and 9, respectively, except for the fact that electrodes 5| and 52 having a uniform spacing between the turns thereof have been replaced by electrodes 6| and 62, respectively, each of which forms a helix having a variable pitch.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without depart= ing 'from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: l v

1. A high-frequency signal-translating stage comprising vacuum-tube electrode structure ef-" -fectively including a'plurality of electron-discharge .paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others of-said discharge paths comprising input electrodes including a control electrode and a cathode, means for forming a virtual cathode between said control electrode and said cathode, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that saidgconductances are substantially equal in magnitudefor all-operating conditions of said stage.

2. A h gh-frequency signal-translating stage comprising vacuum-tube electrode structure effectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others of said clischarge paths comprising two electrodes and input electrodes including a control electrode di's- ,-posed between said two electrodes, and means for applying a positive potential to each of said two electrodes, whereby a negative conductance exists. between said last-mentioned input electrodes, the spacing of .said electrodes and their normal operating potentials being .so proportioned that said -conductances are substantially equal-in magnitude for alloperating conditions of said stage. I

. 3. Ahigh-frequency 'signal translating stage for operating over a wide range of frequencies comprising vacuum-tube electrode structure effectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprisingconventional electrodesinclud- 'ing input electrodes having appreciable positive conductance therebetween, others of said discharge paths comprising input electrodes including a control electrode and a cathode, means for forming a virtual cathode between said control electrode and said cathode, whereby a negative transconductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are approximately equal in magnitude over the frequency range ofsaid stage.

4. A high-frequency signal-translating stage comprising vacuum-tube electrode structure effectively' including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes. having appreciable positive conductance therebetween, others of said discharge paths comprising input electrodes includ- .ing a control electrode and a cathode, means for forming a virtual cathode between said control electrode. and said cathode, whereby a negative conductance exists between said last-mentioned input electrodes, said input electrodes being adapted to be operated over a wide range of bias potentials, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are complementary over the grid-bias range of said input electrodes.

5. A high-frequency signal-translating stage comprising vacuum-tubeelectrode structure effectively including a plurality of electron-discharge paths; certain of said discharge paths comprising conventional electrodes including input and output electrodes and having appreciable positive conductance between said input electrodes; others of said discharge paths comprising two electrodes, input electrodes including a control electrode disposed between said' two electrodes, and output electrodes; means for applying a positive potential to each of said ,two elec-' trodes, whereby a negative transconductance exists between said lastmentioned input electrodes; said two sets of input electrodes and said two sets of output electrodes being respectively coupled in parallel in said stage; and the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude for all crating conditions of said stage.

6. A igh-frequency signal-translating stage comprising vacuum-tube electrode structure efiectively including a plurality of electron discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including a cathode and a control electrode havingan appreciable positive conductance therebetween, others 01 said discharge paths including two electrodes, a cathode, and a control electrode disposed between said two electrodes,- means for applying a positive potential to each of said two electrodes, means for biasing each of said control electrodes negatively with respect to its associated cathode, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude for all operating conditions of said stage.

' 7. A high-frequency signal-translating stage comprising vacuum-tube electrode structure eifectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therebetween, others of said discharge 'paths comprising two electrodes and input electrodes including a control electrode disposed between said two electrodes, means for applying a positive potential to each of said two electrodes, whereby anegative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are approximately equal in magnitude, whereby said conductances vary as substantially the same function of frequency over the operating range of said stage and the effective input conductance to said stage is substantially zero over said operating range.

8. A high-frequency signal-translating stage comprising vacuum-tude electrode structure effectively including a plurality of electron-discharge paths .in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others of said dispotentials being so'proportioned that. said conductances are approximately equal in magnitude, whereby said conductances vary between the limits of 1.5 and 2.0 power of the frequency over the operating-range of said stage and the efiective inp'ut conductance to said stage is substantially zero over said operating range.

9. A high-frequency signal-translating stage comprising vacuum-tube electrode structure effectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive transconductance therebetween, others of said discharge paths comprising two electrodes and input electrodes including a control electrode disposed between said two electrodes, means for applying a positivepotential to each of said two electrodes, whereby a negative conductance exists between said last-mentioned input electrodes, said input electrodes being operable over a wide range of bias voltages, the spacing of said electrpdes and their normal operating potentials being so proportioned that said conductances vary substantially as the same function of said bias voltages over the operating range of. said stage, whereby the effective input conductance to said stage is substantially zero over said operating range.

' 10. A high-frequency signal-translating stage comprising a single vacuum-tube electrode structure eflectively including a plurality of electrondischarge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therebetween, others of said cathode between saidcathode and said control electrode, whereby anegative conductance exists between said cathode and said control electrode,

the spacing of said electrodes and their normal comprising a common cathode, certain of said operating potentials being so proportioned that said conductances are substantially equal in magnitude for all operating conditions of said stage. 11. A high-frequency signal-translating stage 5 comprising a single vacuum-tube electrode structure efiectively including a plurality of electrondischarge paths in parallel, certain of said paths comprising conventional electrodes including input electrodes having an appreciable positive con- 10 ductance therebetween, others of said discharge paths comprising two electrodes and input'electrodes including a control electrode disposed be tweensaid two electrodes, and means for applying a positive potential to each of said two elec? 15 trodes, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in mag- 2 nitude for all operating conditions of said stage. 12. A high-frequency signal-translating stage comprising a single vacuum-tube electrode structure eflectively including a plurality of electrondischarge paths in parallel, said discharge paths discharge paths comprising conventional electrodes including input electrodes comprising a control electrode, said input electrodes having appreciable positive conductance therebetween, others of said discharge paths comprising two electrodesand input electrodes including a cathode and said control electrode, said control electrode being spaced between said two electrodes and the one of said two electrodes between said control electrode and said cathode having an efl'ective area which is appreciably smaller than said control electrode, thereby being effective only upon said others of said discharge paths, means for applying a positive potential to each of said two electrodes, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude for all operating conditions of said stage.

ROBERT L. FREEMAN. 

